The invention will be described with reference to a glucose biosensor but it should be understood that this use is illustrative only and the invention may be applied to other types of sensors or systems based on electrochemical cells e.g. high temperature filters, display items, instruments for chemical analysis, for example of heavy metals in waste water or the like.
Electrochemical glucose analysers such as those used by diabetics to monitor their blood sugar levels and in clinics or hospitals are based on reaction pathway (a): ##STR1##
Typically, a sensor within an analyser comprises of a mediator which transfers electrons between an enzyme and an electrode as shown in reaction pathways (b) above. The signal produced upon such a transfer is processed and expressed as a function of the enzyme substrate, for example, the use of glucose oxidase (GOD) for the Measurement of glucose in blood or serum.
A known glucose sensor is a strip on one end of which is a predefined target or sample area and having three electrodes each attached lengthwise on the strip and across the target area. A silver chloride electrode in the middle forms a pseudo-reference electrode and the remaining two are known as working electrodes. Each working electrode is formed of a core of carbon paste. One of these electrodes is coated with a layer of GOD and mediator and the other with a layer of mediator such as ferrocene.
A drop of blood or sample amounting to about 25 .mu.l is applied onto the target area and the strip is then inserted into a detector which measures the current at each of the working electrodes. This current corresponds to the oxidation of the ferrocene, and (at the working electrode containing GOD) the reoxidation of ferrocene produced by the reduction of mediator due to the reaction pathway described above. The difference in these currents is used to estimate the original glucose concentration in blood.
EP 289 269 describes a biosensor comprising a base sensor strip and an overlying exclusion layer or membrane which excludes red blood cells or regulates the flow of sample onto the sensor strip.
In WO 93/13408, a similar arrangement wherein a composite membrane welded to a housing containing electrodes is disclosed. The composite membrane consists of a porous membrane with an immobilized catalytic agent and at least one of a protecting or blocking membrane. The catalysed product flows from the composite membrane into an aperture defined within the housing to interact with the electrodes.
EP 230 472 discloses a perforated body such as nylon non-woven fabric positioned above an insulative base on which three electrodes are screen printed. A sample is applied above the perforated body and passes through it onto the base where an enzyme catalysed reaction occurs.
U.S. Pat. No. 4,431,507 discloses an electrode arrangement in which two working electrodes are dispersed on opposite sides of a porous membrane, one to prevent interfering materials contained in the test solution from being oxidized by the other working electrode.
The electrode arrangement is used in a cell having a buffer solution in which a counter and reference electrode are suspended.